444 THE OPERCULUM. 



indeed, between the operculum and the valve of a shell con- 

 sists in the former having no cavity, the cone of which it is 

 formed being either very much depressed, so as to become 

 nearly flat, or even concave, as in the annular or some sub- 

 annular opercula, or very much compressed, forming only a 

 spiral riband, as in the spiral ones. Opercula are never 

 attached to their shells by ligaments or by any other means 

 than that of the adductor muscle ; and they are always free, 

 except in the genus Capulus, which offers a remarkable 

 anomaly in this part being immediately attached by its outer 

 surface to other marine bodies. 



It is proper to observe, that in describing the operculum 

 I have called that the anterior margin, which is nearest to 

 the pillar of the shell, as when the animal is walking this 

 part is directed towards its head ; and the right and left 

 extremities are the parts corresponding with the right and 

 left sides of the body. When the operculum is placed in 

 this position the left end is that which fits into the front, 

 and the right that which corresponds to the hinder part, of 

 the mouth. This distinction of parts may, at first sight, be 

 thought trifling ; but to the practical zoologist its import- 

 ance is very great. The position of the nucleus of the 

 operculum is, for example, almost the only conchological 

 character by which four distinct genera of shells can be 

 divided into groups, although their animals differ both in 

 external form and in internal anatomy. In the genera Bithy- 

 nia and Paludina, which have the nucleus of the oj)erculum 

 nearly central, the animal has short tentacles and no air bag, 

 whilst in Ampullaria and Ceratodes, which have the nucleus 

 of the operculum on its anterior side, the animals have very 

 long tentacles, and a large air pouch by the side of the 

 branchiae. 



Opercula may be divided into three very distinct kinds, 

 according to their form and manner of growth, and these 

 may be again subdivided according to the mode in which 

 they are covered with various deposits. 



The annular operculum (Fig. 78) may be considered the 



most simple, the very depressed cone ot 



lg ' 78 ' which it is formed being nearly regular, 



J-- - : s,, with the apex more or less central, and the 



coat of new matter, by which it is increased 



in size, forming complete rings round its 



circumference ; in which particulars it may 



be compared to the simple conical shells of 



the genera Patella, Fissurella, &c. This 



kind of operculum does not alter its place in the mouth, 



